The surface of porous silicon dioxide can be chemically modified in accordance with various known methods in a manner which affects the adsorption properties thereof. For this purpose, mono- and/or polymolecular layers of certain groups which for the most part are functional are bound to the siloxane skeleton of the silicon dioxide by an Si--O--C--, Si--C--, or Si--N-- bond. Thus, in correspondence with the chemical structure of the groups, modified silicon dioxides are obtained having specific adsorption properties. In this connection, basically the following methods are known:
1. Reaction of silicon dioxide with monomeric organosilicon compounds, thus introducing corresponding organosilyl groups which cover the surface of the SiO.sub.2 in a monomolecular layer (c.f., e.g., K. Unger, Angewandte Chemie, Volume 84 (1972), page 331).
2. Substitution and/or polymerisation reactions involving the groups introduced according to (1) to produce polymolecular layers (c.f., e.g., O.E. Brust et al., Journal of Chromatography, Volume 83 (1973), page 15, and German Unexamined Laid-Open Application No. 2,309,129).
3. Reaction of silicon dioxide with oligomeric or polymeric organosiloxane compounds, which are chemically bound to the siloxane structure to produce layers with thicknesses of 30-2,000 A (c.f., e.g., German Unexamined Laid-Open Application No. 2,125,428).
All of these processes have a number of disadvantages which manifest themselves, in particular, when using the products thereof as carrier materials in chromatography or as catalysts.
In the reaction of silicon dioxide with organosilicon compounds, unreacted hydroxyl groups remain or new ones are simultaneously produced. Due the presence of such polar adsorption centers, the adsorption characteristic of the modified products is adversely affected. Furthermore, the silicon dioxide products modified by surface reactions often additionally contain physically adsorbed proportions of reaction products which, in part, desorb during subsequent use of the modified materials. In the case of polymeric layers on silicon dioxide, the adhesive strength of these layers, which is greatly dependent on the reaction conditions employed, is frequently impaired. Moreover, the porous layers are capable of swelling and are sensitive to changes in the pH of solutions with which they are in equilibrium.
It is an object of this invention to provide a process for producing, in a very simple manner, organically modified silicon dioxides which avoids the aforedescribed disadvantages.